If the Los Angeles premiere of “Saving Mr. Banks” served as a glamorous red carpet reunion for some of Hollywood’s most storied stars — Julie Andrews, Dick Van Dyke — the New York city premiere was more like an insider happy hour.

Emma Thompson, who plays “Mary Poppins” author P.L. Travers in the film opposite Tom Hanks’ Walt Disney, cozied up in a booth at midtown’s Monkey Bar on Wednesday evening with director Alison Owen and writer Kelly Marcel. The three sipped cocktails and chatted about the film with guests before a screening at the Museum of Modern Art.

Ms. Marcel says that although she was writing a script for Disney that featured the real Walt Disney as a character, she didn’t sugarcoat her work.

“I don’t censor myself, and I certainly didn’t censor him,” she said. “So I was prepared to go to bat.”

When the company returned her script with no changes, Ms. Marcel said she was speechless. “I blinked for like, two minutes. It was very brave of them.”

There were things she had to handle delicately, she said, like Walt Disney’s smoking habit. “Existing contracts meant we couldn’t portray him literally inhaling, so we got as close as we could,” she said.

For example, even though Walt Disney was a heavy smoker, his character could not be seen inhaling in the film. He died of lung cancer in 1966.

“But believe me, he smokes,” she said. “You see him holding a cigarette, putting out the ashes, coughing, the whole bit. We don’t pretend it wasn’t happening.”

“Saving Mr. Banks” follows uptight author P.L. Travers on a visit to Southern California in 1961 during which Mr. Disney tries desperately to obtain the film rights to “Mary Poppins.” A prickly grouch, Ms. Travers isn’t fond of the fantastical Disney brand (or Mr. Disney, for that matter) and their personalities clash incessantly. Ms. Marcel said one of her biggest challenges was finding ways to bring out their colorful personalities without making them into caricatures.

“They were rather extreme people, stubborn, damaged, a bit larger than life, ” she said. “You have to make sure they’re three-dimensional or they become cartoons.”

To accomplish this, she added secondary relationships to bring out the complexity of each character, particularly Ms. Travers: Colin Farrell plays her alcoholic but loving father in 1907 and Paul Giamatti portrays her soft-hearted, fictional chauffeur in 1961.

Ms. Marcel said she recently finished the much-anticipated erotic script for “Fifty Shades of Grey,” due out in February, 2015, and is about to begin working on a script for a film about Winston Churchill’s depression, adapted from the 2011 book “Mr. Chartwell” by Rebecca Hunt.

“I know, I’m all over the place,” she said about the variety in tone of her recent projects. “But it keeps me on my toes.”